Remove from gemspec development deps not required to run tests

per discussion in https://github.com/troessner/reek/pull/226

The gems moved into the Gemfile group :local_development may be
used while developing the gem, but are not required for running tests.

Now, one can run the tests without installing all the development gems with
  bundle --without local_development
  bundle exec rake

The choice of 'local_development' has to do with how Bundler brings
evaulates the gemspec development dependencies as both development
and test dependencies.

Also, make explicit the command to run tests on Travis CI